The two broad approaches commonly used in the laboratory for studying the fluid catalytic cracking process are continuous processing units and batch processing units. The continuous processing units are basically scaled-down versions of commercial operating units and are typically very complex systems that are expensive to construct, operate, and maintain. In addition, they require large samples of catalyst as well as feed compared to batch laboratory cracking units. Batch processing units use a single charge of catalyst (typically less than 200 gms.) and process a small sample mass of feed that is usually injected over the catalyst for a period of time on the order of a minute. The ratio of the catalyst mass to the feed mass is referred to as the catalyst-to-oil ratio and typically ranges from 3 to 10. Batch processes provide considerable cost and speed advantages over continuous units for laboratory studies, primarily because of their relative simplicity and the smaller scale.
Two types of batch processes are commonly used: fixed bed and fluidized bed. Fixed bed type units are not appropriate for processing high boiling range feeds, which are often the feed for commercial fluid catalytic cracking units (FCCU). For this and other reasons, many laboratories have abandoned the use of fixed bed reactors for evaluating commercial FCCU operation. Thus, fluidized bed reactors are preferred over fixed bed reactors for studying the fluid catalytic cracking process on a laboratory scale.
One of the most important parameters in fluid catalytic cracking is the time that hydrocarbons are in contact with catalyst. Research over the past several years uncovered that as much as 90% of the feed conversion takes place in the short contact time condition in the feed and catalyst mix-zone of the riser reactor in commercial FCCUs. This knowledge led to the revamp of older and larger reactors to smaller designs, because contact time dramatically affects yields and performance. Normally, reducing the contact time requires that the commercial unit operate at a higher catalyst-to-oil ratio than when the contact time is longer. It is therefore important for laboratory scale fluidized catalytic cracking apparatus to provide the flexibility to vary contact time and simultaneously operate at high catalyst-to-oil ratios.
There are several ways to vary contact time in laboratory fluid-bed reactors. The widely known techniques of altering the hydrocarbon feed rate, the rates of any diluent gases, and/or altering the catalyst charge provide results, however, they are not entirely consistent with commercial experience or have other deficiencies which limit their applicability.
Walsh, U.S. Pat. No. 4,419,328, discloses a laboratory apparatus for investigating the performance of catalytic cracking catalyst utilizing batch techniques and a fluidized bed reactor. Walsh teaches the use of the laboratory apparatus and techniques for obtaining cracking data but not a reactor apparatus or method that can be utilized to emulate the performance characteristics expected in a commercial scale reactor. Walsh discloses a fluidized bed reactor but it does not include a movable feed injector or disclose in any way the important aspects pertaining to injector location and its relationship to controlling hydrocarbon contact time. Walsh does not disclose or teach the injection of multiple feeds at different locations. In addition, Walsh does not disclose or teach an apparatus or method to achieve the catalyst circulation pattern within the reactor including its relation to commercial catalytic cracking as well as ways to enhance the circulation by proper reactor design and injector positioning.
It is well known that fluidized bed reactors of many different designs often exhibit a preferred flow of solids up the center core and return of solids down the normally cylindrical containing wall. Perry's Chemical Engineers' Handbook (Copyright 1984) mentions this tendency (Section 20 pg. 66) and the effect is noted in U.S. Pat. No. 5,262,104-Schwartz. In these references, however, there is no recognition of the potential applications of the circulation pattern nor are any ways to enhance the circulation provided.